Aim to answer every question before checking. Missed questions point you to the sections you need most.
1. (12.1) An allele is:
A. Two alleles (one from each parent) exist at each autosomal locus in diploids. B describes a locus, C describes phenotype.
2. (12.1) A homozygous organism at a locus has:
C. Homozygous (AA or aa) has two identical copies; heterozygous (Aa) has two different alleles.
3. (12.1) Mendel's law of segregation states:
B. Homologs separate at anaphase I, ensuring only one allele per gene per gamete. Fertilization then restores the diploid pair.
4. (12.1) Mendel's law of independent assortment states:
D. Independent assortment explains how homologs pair and randomly orient at metaphase I. Linked genes (close on one chromosome) violate this expectation.
5. (12.2) The expected genotypic ratio from a monohybrid cross Aa × Aa is:
A. The 1:2:1 genotypic ratio collapses to a 3:1 phenotypic ratio when one allele is fully dominant.
6. (12.2) A testcross reveals an unknown genotype by crossing it with:
C. Only recessive alleles come from the tester; any dominant phenotype in the offspring must come from the unknown, revealing its genotype.
7. (12.2) In Aa × Aa, the probability of a homozygous-recessive child is:
B. Probability of receiving a from one parent (
21) × from the other (
21) =
41.
8. (12.3) Incomplete dominance produces offspring with:
D. Heterozygotes produce about half the normal gene product, giving a phenotype between the two homozygotes.
9. (12.3) Codominance occurs when:
A. Codominance shows both alleles at once (type AB carries both A and B surface antigens). Incomplete dominance blends; codominance displays.
10. (12.3) An allele that kills the homozygote but not the heterozygote is called:
C. Heterozygotes carry the allele silently. Crosses between carriers often yield unexpected phenotypic ratios because homozygous lethals do not survive.
11. (12.4) X-linked recessive traits are more common in males because:
B. Males (XY) have only one X, so there is no "good copy" to mask a recessive allele. Classic examples: hemophilia A, red-green color blindness.
12. (12.4) Two unaffected parents have an affected child. For an autosomal recessive trait, this indicates:
D. Trait "skips generations" is a hallmark of autosomal recessive inheritance. Each parent must carry the recessive allele.
13. (12.5) Genes located on the same chromosome are:
A. Linkage is Mendel's fourth exception. Closer loci recombine less often, so they are inherited together more frequently.
14. (12.5) The frequency of crossover between two linked loci is:
C. Two very distant loci on the same chromosome recombine freely and behave like loci on separate chromosomes (50% recombination).
15. (12.5) One centimorgan (cM) is defined as:
B. Map distances are measured in cM: 10 cM ≈ 10% recombination. The human genome averages about 1 cM per 1 Mb of DNA, but this varies by region.
16. (12.6) A point mutation that changes one amino acid in a protein is called a:
D. Silent = same amino acid (synonymous codon). Missense = different amino acid. Nonsense = stop codon. Frameshift = insertion/deletion shifting reading frame.
17. (12.6) A nonsense mutation:
A. The truncated protein is usually non-functional and often degraded via nonsense-mediated decay of its mRNA.
18. (12.6) Genetic drift refers to:
C. Drift is sampling error across generations. In small populations it can fix or eliminate alleles regardless of fitness.
19. (12.7) Hardy-Weinberg equilibrium requires:
B. If all conditions are met, allele and genotype frequencies stay constant across generations. Any violation drives evolution.
20. (12.7) In the Hardy-Weinberg equation p² + 2pq + q² = 1, the term 2pq represents:
D. p² = AA, q² = aa, 2pq = Aa. For autosomal recessive diseases, the carrier rate 2pq is often much higher than the disease rate q².
21. (12.7) The founder effect occurs when:
A. The founder effect is a special case of genetic drift. Classic examples: Ellis-van Creveld among the Amish, Huntington's cluster in Venezuelan Lake Maracaibo.
22. (12.7) Disruptive selection favors:
C. Disruptive selection can split one population into two, contributing to sympatric speciation. Stabilizing selection (A) and directional selection (B) are the other two modes.
23. (12.4) A hemophiliac father (X-linked recessive) and an unaffected non-carrier mother will produce:
B. Daughters get the father's Xʰ and the mother's normal X = carriers. Sons get the father's Y (no trait) and the mother's normal X = unaffected. X-linked fathers cannot pass the allele to sons.
24. (12.7) Speciation is:
D. Allopatric (geographic barrier) and sympatric (within the same range) speciation both require the two groups to stop exchanging genes, letting their gene pools diverge.
